1. Field of the Invention
In one of its aspects, the present invention relates to a mold for producing a molded element, preferably a molded foam seat element, more preferably a molded foam vehicular seat element. In yet another of its aspects, the present invention relates to a method for manufacture of a mold.
2. Description of the Prior Art
Many articles are manufactured by placing a raw material into a cavity in a mold wherein the raw material undergoes a physical change (e.g., it expands or foams) and the article produced thus acquires the shape of the cavity. In particular, this technique is commonly employed for producing foamed articles made from polymeric foams such as polyurethane foam, latex (e.g., natural and styrene-butadiene rubber) foam and the like.
For example, automotive seats are commonly manufactured from polyurethane cushions which are molded to shape and then covered with a vinyl, cloth or leather finish cover (also known as a “trim cover”). Polyurethane foams are somewhat unique in that foaming and at least a portion of the polymerization process occur simultaneously. Thus, in the production of polyurethane foam using, for example, a conventional cold foam technique, a typical formulation comprises:                1. polyol (and/or other active hydrogen-containing material);        2. water;        3. catalyst;        4. cross-linking agent; and        5. polyisocyanate.        
The mixture is dispensed into a mold using a suitable mixing head, after which the mold is then closed to permit the expanding mass within it to be molded. Accordingly, it is convenient generally to refer to the mixture initially dispensed into the mold as “a liquid foamable polymeric composition” or, in this case, “a liquid foamable polyurethane composition”. As the composition expands in the mold, polymerization occurs and the polymer so formed becomes solidified.
When molding a liquid foamable polymeric composition to form articles, such as polyurethane foam articles, it is conventional to use a clam-shell mold comprising a bottom mold (also referred to as a “bowl”) and a top mold (also referred to as a “lid”) which, when closed, define a mold cavity. In a conventional process, the mold is opened, the liquid foamable polyurethane composition is dispensed into the mold cavity and the mold is closed as a chemical reaction causes the composition to expand. After the mold is closed, the composition expands to fill the interior cavity of the mold. Alternatively, the composition may be dispensed into a closed mold. In either case, as the polymerization reaction is completed, the foam cures and permanently assumes the shape of the mold cavity.
As is known to those of skill in the art, it is important during this process that the mold be adequately vented to allow the air present in the mold to exit the mold as the foamable composition expands. Further, it is important to allow a portion of the gases (typically CO2 in the production of polyurethane) generated during polymerization to exit the mold.
Failure to adequately vent the mold results in defective molded articles exhibiting symptoms of improper foaming such as surface hardening (or foam densification) and/or void formation in the finished article due to trapped gas or air bubbles. At the other extreme, excess venting of the mold will also result in defective molded articles due to collapse of the foam prior to curing; this phenomenon is often referred to as the “soufflé” effect. Thus, proper venting of a mold is an important factor in producing molded articles of acceptable quality.
Typically, first generation clam-shell molds have been designed with drilled or cut passages in the top mold to provide vents. Locating, sizing and deciding upon the number of these vents is a matter of some skill on the part of mold designer and the production engineers, and is often an iterative procedure with more vents being added to various locations or other vents being blocked-off after test runs have been made.
During molding operations some liquid foamable polymeric composition which moves into the vent is wasted. It is generally desired to minimize the amount of wasted material (also known as “flash”, “mushrooms”, “buds”, “pancakes” and the like) for two reasons, namely (1) the wasted material adds to the overall expense of chemicals required to produce the finished article, and (2) the wasted material must be removed from the molded article prior to the finish cover being applied, thereby necessitating additional labour and the costs associated therewith.
In U.S. Pat. No. 5,356,580 (Re.36,413), U.S. Pat. No. 5,482,721 (Re.36,572) and U.S. Pat. No. 5,587,183 [collectively referred to as “the Clark et al. patents”], there is disclosed a second generation mold. The second generation mold taught by the Clark et al. patents included an improved parting line vent (also known in the art as a “ribbon vent”). This improved parting line vent is a highly efficient vent that facilitates the bulk of venting of the mold cavity.
In copending U.S. patent application Ser. No. 10/973,985 [Cathcart et al.], there is taught a third generation mold for producing foam articles. In a preferred embodiment, the mold comprises a lid and a bowl releasingly engageable to define a mold cavity, the lid comprising: (i) a vent having a passageway for gas to escape from the mold cavity, and (ii) a plurality of grooves connected to the vent. The use of a plurality of grooves/slots in the mold cavity surface effectively acts as a siphon to draw gas away from the composition to be molded. The plurality of grooves/slots may be connected to one or more parting line vents which allow for escape of the gas from the mold cavity to the exterior of the mold.
Thus, over the years, there has been much attention devoted to improving venting, particularly parting line or part-line venting in clam-shell molds. The overall goal has been to produce so-called trim-free parts—i.e., parts have no trim or have minimal trim that need not be removed. The advantages of such an approach include a significant reduction in labor and in production material waste.
What has received relatively little attention is the remaining portion of the parting line or part-line—i.e., the portion not containing the vents. Typically, this remaining portion is simply two flat surfaces disposed horizontally or at an angle (e.g., 15°) with respect to the mold cavity. The width of the part line is typically 25 millimeters or more. This can result in the production of uncontrolled flash that requires trimming notwithstanding the fact that vents disposed in the parting line or part-line are designed to avoid the need for trimming. In other words, the advantage of utilizing a vent that is capable of producing so-called trim-free parts can be neutralized by the uncontrolled production of flash material in the areas in the parting line not containing the vents.
Accordingly, there remains a need in the art for an improved mold having a parting line or part-line which obviates or mitigates the production of uncontrolled flash, particularly in areas of the parting line or part-line which do not contain a vent.
It would be particularly advantageous if such an improvement could be implemented without the requirement for large capital expenditure.